
Make the mechanism more general, so that other tests can now specify overrides for various defaults, if needed. This replicates what GLib does: https://gitlab.gnome.org/GNOME/glib/-/blob/main/glib/tests/meson.build#L475. It introduces no functional changes in how the current set of tests run. Signed-off-by: Philip Withnall <pwithnall@gnome.org> Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3605>